At present, for commercially obtaining hydrogen in wide areas, the production of hydrogen by reforming of organic compounds, for example, hydrocarbons such as methane and propane, and alcohols such as methanol, particularly by steam reforming thereof is excellent. However, the hydrogen gases obtained under practical reforming conditions by these methods contain several per cent of carbon monoxide. Methods for further converting carbon monoxide to hydrogen and carbon dioxide by the modification reaction or the shift reaction with water vapor have been known. However, from both the chemical equilibrium and the activity of conventional catalysts used, there is a limitation on a reduction in the content of carbon monoxide, and actually, the content of carbon monoxide can only be reduced to about 1%.
On the other hand, attempts to use the hydrogen-containing gases as fuels for fuel cells have been made. In that case, however, it is particularly required to reduce the concentration of carbon monoxide to several ppm or less for allowing PEFCs to efficiently work at low temperature. The reason for this is that carbon monoxide acts as catalytic poison to electrode catalysts of the fuel cells, and that the catalytic poison action of carbon monoxide to the electrode catalysts becomes significant as temperature is lowered. With respect to this, the electrode catalysts have also been improved to raise the resistance to carbon monoxide. For example, a technique has been reported in which a platinum-ruthenium alloy is used as the electrode catalyst to provide the resistance to carbon monoxide. In this electrode catalyst, however, the catalytic poison action of carbon monoxide does not appear when the concentration of carbon monoxide contained in the hydrogen-containing gas is 100 ppm or less. At that time, the working conditions of the PEFC is limited to a high temperature of 100.degree. C. or more.
Further, it has been suggested that the addition of 6% to 13% of oxygen to a hydrogen-containing gas containing carbon monoxide allows a PEFC to work without a decrease in voltage of electric power generated from the PEFC. However, the addition of such a large amount of oxygen brings about the danger of a gas explosion and results in remarkable non-electrochemical oxidation of hydrogen at an electrode to cause a large loss of hydrogen. Further, a wide temperature distribution is developed on a surface of the electrode to cause a significant decrease in voltage generated. Further, it is also reported that if the concentration of carbon monoxide contained in a hydrogen-containing gas is 100 ppm or less, the amount of oxygen added to the hydrogen-containing gas supplied to the above-mentioned electrode requires only about 0.4%. However, it is necessary to provide a step for previously decreasing the concentration of carbon monoxide to 100 ppm or less, so that the whole system becomes complicated. Moreover, even in this case, the non-electrochemical oxidation of hydrogen at the electrode can not be avoided because of the existence of a slight amount of oxygen, which causes the enlargement of a temperature distribution on a surface of the electrode and a decrease in voltage of a fuel cell.
Further, a method has also been studied in which oxygen-containing gas is added to a hydrogen-containing gas containing carbon monoxide, and this gas is brought into contact with an oxidative removing catalyst, thereby oxidizing carbon monoxide to remove it. According to this method, no load is applied to a fuel cell complicatedly operated. This method is therefore an excellent method, if an effective oxidation reaction catalyst is present. According to the report of Toyota Motor Corp. (The Second Fuel Cell Symposium Lecture Proceedings, page 235, 1995), it is reported that the concentration of carbon monoxide is decreased to a limit concentration of detection or less by the removal of carbon monoxide by oxidation using a ruthenium catalyst at a reaction temperature of 100.degree. C. In this report, however, the limit concentration of detection of carbon monoxide is 20 ppm. It is further reported that as a result of the reaction at 80.degree. C., 150 ppm of carbon monoxide remains. This shows that the activity of the oxidation reaction catalyst is insufficient at low temperature. No reaction example at lower temperature is described.
Recently, the utilization of PEFCs as power sources for vehicles have been studied. When used for vehicles, the PEFCs and devices for producing the hydrogen-containing gases used as fuels thereof are complicatedly operated and stopped, and rapid standup is required in starting. When the activity of the catalyst for removing carbon monoxide is low, it is necessary to always heat the device at a specified temperature required by the catalyst. Accordingly, a large amount of energy becomes necessary. In some cases, it is also necessary to always heat the PEFC in which the electrode is poisoned by carbon monoxide at low temperature. Further, the carbon monoxide removing reaction and the reaction in the PEFC are exothermic reactions, so that devices for cooling and heating are required, unfavorably resulting in a complicated system.
According to JP-A-8-295503 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), an attempt is made to remove carbon monoxide at a relatively high temperature of 100.degree. C. or more, preferably 100.degree. C. to 300.degree. C., using a ruthenium catalyst in which ruthenium is carried on titanium oxide. However, no example at a temperature of lower than 100.degree. C. is reported.
There are many reports about studies of adsorption of carbon monoxide on ruthenium catalysts or the oxidative removing catalyst, and further about the use of the pulse process (Shokubai Koza (Catalyst Course), vol. 2, page 160, edited by Shokubai Gakkai (Catalytic Society), Kodansha (1985); Catalysts, 12, 1 (1970); Catalysts, 23, 483 (1981)). However, there is no report in which the relationship between the selective oxidation reaction of a slight amount of carbon monoxide contained in hydrogen gas and the adsorption of carbon monoxide on a catalyst is described, and no indication for selecting an effective catalyst is established.
As described above, carbon monoxide contained in hydrogen gas reduces the output voltage of the fuel cells, so that the exhaustive reduction of carbon monoxide is required for the high efficiency operation of the PEFCs. The greater reduction of carbon monoxide is required for the operation at low temperature, which is an outstanding feature of the PEFCs. Accordingly, the realization of higher activity and selectivity of the oxidative removing catalysts for carbon monoxide at low temperature is indispensable, and particularly, the high activity and selectivity at a temperature of 100.degree. C. or less, more preferably 80.degree. C. or less are required. When various starting conditions and operating conditions are considered, the activity at room temperature or at a temperature of 0.degree. C. or less becomes important.